1. Field of the Invention
The present invention relates generally to an inter-exchange multiplexed communication system for performing multiplexed communication between a plurality of exchanges. More particularly, the invention is concerned with a communication system suited advantageously for a call connection control between a pair of private branch exchanges each including a multiplexing/demultiplexing unit.
2. Description of the Related Art
With remarkable progress in the digital signal techniques in the recent years, it is now possible to carry out the ordinary voice telephone communication at a compressed transmission rate in a range of 8 to 32 kbps. In the communication between data terminals, a transmission rate on the order of several kbps tends to be employed increasingly. In contrast, in the case of an integrated services digital network (hereinafter referred to as the ISDN network), the minimum unit for the circuit switching is a 64-kbps data channel. Accordingly, when the ISDN network is used for the telephone and data communication there arises a large amount of waste.
In this conjunction, there is disclosed in JP-A-61-61543 (Japanese Patent Application Laid-Open No. 61543/1986) an interface system capable of accommodating simultaneously a voice terminal and a data terminal in a single subscriber's line in which a voice call and a data call are transmitted simultaneously at a rate of 64 kbps through multiplexed communication with a view to utilizing effectively the 64 kbps information channel. Further, as a system designed for effective channel utilization in the communication dedicated for digital data, there is proposed such a system in which compression/expansion rules are made to be dynamically controllable on a time-slot basis between an exchange and the counterpart therefor, as is disclosed in JP-A-62-183297.
However, in the case of the prior art systems mentioned above, no consideration is paid to the means for automatically releasing the multiplexed circuit between the private branch exchanges (also referred to simply as PBX) when no multiplexed call exists, as a result of which there arises a problem that when they are interconnected via a network where message rate is metered on a unit time basis, the charging may be made unreasonably. A second problem of the prior art system is seen in the fact that special protocol processing is required between the private branch exchanges in order to allocate the channel of a corresponding bearer rate to the call taking place.